This invention relates to an electrostatic micromotor comprising a rotor having a hub and a peripheral region, a stator having coplanar electrodes disposed opposite the peripheral region and intended to produce an electric field between the stator and the rotor, and means for rotatably connecting the rotor's hub to the stator.
All micromotors about to be mentioned hereinafter, including the micromotor according to the invention, are manufactured in plates made of silicon or another material having similar characteristics by means of methods derived from those that are used to manufacture integrated circuits, and which are well known.
Methods for the manufacture of such micromotors are described for example in U.S. Pat. No. 4,740,410, in the report of a lecture entitled "IC-Processed Electrostatic Micromotors" given by L. S. Fan et al. at the IEEE Integrated Electrical Devices Meeting held from 11 to Dec. 14, 1988 in San Francisco, USA, and in the report of a lecture entitled "IC-Processed Micromotors Design, Technology and Testing" given by Y. C. Tai et al at the IEEE Micro-Electro-Mechanical Systems Meeting held in Salt-Lake City, USA, from Feb. 20-22, 1989.
These methods will therefore not again be described here, even if they may differ slightly from one another on the particular construction adopted for the micromotors.
It will be merely remembered that all these methods comprise a step of etching a sacrificial layer with a suitable etching liquor for severing the rotor of the motor from its stator and allowing that rotor to freely turn with respect to that stator.
The above reports disclose micromotors that comprise a flat cross-shaped rotor having four or eight arms joined by a central hub and a stator having six or twelve radial control electrodes arranged in a circle around the rotor and in the same plane as the latter.
The means for mounting the rotor on the stator include a cylindrical opening provided in the central hub and a pivot forming part of the stator and fitted into the opening.
To set the motor's rotor in motion, its arms are each subjected to a tangential force produced by an electrical field set up by a voltage applied to selected control electrodes.
But this electric field also subjects the rotor's arms to radial forces. If the ends of these arms were all strictly equidistant from the control electrodes, these radial forces would compensate each other in pairs and their resultant on the rotor would be nil.
But it is of course impossible to manufacture such a motor that satisfies this condition since, for one thing, there must be some play between the rotor's hub and the stator's stationary pivot for the rotor to be able to rotate about this pivot.
The distances between the ends of any pair of diametrically opposite arms and the control electrodes are thus always different and the radial forces which are exerted on these arms and which are inversely proportional to the square of these distances, do not compensate each other. The rotor is thus also subjected to a resultant radial force that causes the inner surface of its hub's opening to rub against the stationary pivot that extends therethrough.
The resulting friction obviously reduces the motor's useful torque, possibly even to the extent of preventing its rotor to rotate. Further, the effect of such friction cannot be compensated by increasing the voltage applied to the control electrodes because such an increase would have the effect of increasing the radial force exerted on the rotor and hence also to increase the above friction.
A number of motors free of this drawback are described in an article entitled "Harmonic Electrostatic Motors" authored by W. Trimmer and R. Jebens and published in issue 20 (1989) of Nov. 15, 1989 in the journal "Sensors and Actuators" published by Elsevier Sequoia in the Netherlands.
One of these motors, diagrammatically illustrated in FIGS. 4 and 5 of this article, comprises a planar stator provided with control electrodes and having a rotor in the form of a solid circular disc which, in the absence of voltage on the stator's electrodes, is also planar and is moreover parallel to the stator.
The means for mounting this rotor on the stator comprise a pivot forming part of the rotor and necessarily, although not shown, a bearing fixed to the stator, in which the pivot can rotate.
The stator's electrodes are successively and cyclically supplied in such a way that all points of the rotor's periphery come into contact seriatim with all points of the stator located on a circle having a diameter less than the rotor's diameter.
As a result, the rotor rotates about its axis in the same direction as that in which the point of contact between the rotor and the stator travels along the above circle
The electrostatic force that attracts a point of the rotor's periphery against the stator is obviously substantially perpendicular to the latter's plane and has therefore practically no radial component, this being an advantage in relation to the other electrostatic motors described above.
But this electrostatic force sets up a tilting torque that tends to cause the rotor's plane to rotate about a straight line perpendicular to the rotor's axis of rotation and to the straight line joining the rotor's center to the point of application of the force.
As this rotor necessarily has some rigidity, this tilting torque also causes the friction between the rotor's pivot and its bearing to increase.
Applicant has now found that, despite the fact that this motor's rotor has been termed "flexible" in the above article, it has, because of its solid disc configuration, a rigidity such that this friction, due to the above tilting torque, is sufficiently great to substantially decrease this motor's torque.
An object of the present invention is to provide a micromotor of a kind similar to the latter, but in which the friction between the moving part and the stationary part of the means for mounting the rotor on the stator is clearly less, whereby the torque supplied by the micromotor according to the invention can be clearly greater, all other things being equal.
To this end the claimed micromotor comprises a rotor having a hub and a peripheral region, a stator including coplanar electrodes opposite said peripheral region for setting up an electric field between said rotor and said stator, and means for rotatably connecting said hub to said stator, wherein said peripheral region is made up of a substantially planar ring substantially parallel to the plane of said electrodes in the absence of said electric field and covering said electrodes at least partially in a plan view of said motor, and wherein said rotor further includes means for mechanically connecting said ring to said hub that are elastically deformable in response to a force applied to said ring in a direction substantially perpendicular to the plane of said electrodes, said hub, said peripheral region and said mechanical connection means defining between them openings extending through said rotor.
The micromotor according to the present invention will now be described hereinafter in detail with reference to the accompanying drawings which illustrate, by way of non-limiting example, several embodiments thereof.